Found that _sqlite_extract (see patch) was returning an integer for month, causing the filter to fail. This was introduced in ChangeSet 8131, which removed str() around the return value. We added unicode(). No one involved is a regular maintainer, so this needs review.

(In [8494]) Fixed #8214 -- Added back a stringification that was dropped during [8131].
This restores filtering on dates in the admin, for example. Thanks to akfarrell
and a small team of workers at the Portland Sprint.